(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for inspection of contact balls coplanarity of singulated device packages whereby contact ball coplanarity inspection is performed immediately after and as part of device singulation and the device pick-and-place processing sequence.
(2) Description of the Prior Art
One of the results of increased circuit packaging density has over the years been the development of methods that provide an increased number of input and output (I/O) points that can be used for the interconnection of semiconductor devices. These devices are divided into a number of classes dependent on the method that is used to provide the increased I/O count, the best known classes within these are the Ball Grid Array (BGA) devices and the Column Grid Array (CGA) devices. A Ball Grid Array (BGA) is an array of solderable balls placed on a chip carrier. The balls contact a printed circuit board in an array configuration where, after reheat, the balls connect the chip to the printed circuit board. BGA""s are known with 40, 50 and 60 mils. spacings in regular and staggered array patterns.
In general, Chip on board (COB) techniques are used to attach semiconductor dice to a printed circuit board, thereby including flip chip attachment, wirebonding, and tape automated bonding (TAB). Flip chip attachment consists of attaching a flip chip to a printed circuit board or other substrate. A flip chip is a semiconductor chip that has a pattern or arrays of terminals spaced around an active surface of the flip chip for face down mounting of the flip chip to a substrate.
Generally, the flip chip active surface has one of the following electrical connectors: BGA (wherein an array of minute solder balls is disposed on the surface of the flip chip that attaches to the substrate); Slightly Larger than Integrated Circuit Carrier (SLICC) (which is similar to the BGA but having a smaller solder ball pitch and diameter than the BGA); a Pin Grid Array (PGA) (wherein an array of small pins extends substantially perpendicularly from the attachment surface of a flip chip, such that the pins conform to a specific arrangement on a printed circuit board or other substrate for attachment thereto. With the BGA or SLICC, the solder or other conductive ball arrangement on the flip chip must be a mirror image of the connecting bond pads on the printed circuit board so that precise connection can be made. The flip chip is bonded to the printed circuit board by refluxing the solder balls. The solder balls may also be replaced with a conductive polymer. With the PGA, the pin arrangement of the flip chip must be a mirror image of the recesses on the printed circuit board. After insertion, the flip chip is generally bonded by soldering the pins into place.
In order to assure reliable soldering of the contact balls to a Printed Circuit Board, the planarity of the points of contact of the contact balls with the PCB must be inspected. These points of contact of the contact balls must essentially form in one plane so that the device can be surface mounted on a PCB by soldering the contact balls to the contact pads that are provided for this purpose in the surface of the PCB. It is known that a planar surface is determined by three points, it therefore follows that generally only three balls of the contact ball array need to contact the underlying surface of the PCB in order to establish contact with this surface. Device packaging reliability requires that the points of contact between a BGA device and the surface of the PCB on which the BGA device is mounted must not be limited to only three points of contact but must be extended such that all contact points of the contact balls rest on the surface of the PCB prior to flowing the contact balls with the contact pads on the surface of the PCB. To assure this planarity of contact points of the contact ball, the device can be manually inspected by placing the device on a smooth surface after which any lack of contact between the contact points of the contact balls and the smooth surface can be readily identified. The characteristic that reflects the planarity of the contact points of the contact balls is the coplanarity of the contact balls which is a measure of the closeness of the contact points of the contact balls to the underlying surface. Good coplanarity is required in order to provide reliable and low resistivity contact between the contact ball of the BGA device and the underlying PCB.
Coplanarity measurements must be made with a degree of accuracy and therefore typically require the use of optical techniques. Most frequently measured in this manner are surface height of the contact points of the contact balls as a function of the geometric location of the contact ball within the array of contact balls. BGA array devices are typically formed using a processing environment that creates multiple BGA devices as sub-assemblies which are, after the BGA device creation is completed, separated or singulated into individual BGA devices. These individual BGA devices are further packaged to form the more complex semiconductor device package. Inspection of contact ball coplanarity typically takes place after the singulation of the devices has been completed thus adding the time that is required for the coplanarity inspection to the processing time of the BGA device. This sequence of device singulation followed by coplanarity inspection therefore has a device throughput that is lower than the device throughput that can be obtained if the inspection for coplanarity can be removed from this processing cycle. The process of the invention provides such a method and, in doing so, increase device throughput for BGA type devices.
U.S. Pat. No. 5,465,152 (Bilodeau) shows a coplanarity inspection method.
U.S. Pat. No. 5,843,808 (Karnezos) teaches a method for automated assembly of packages including singulation. However, this reference differs from the invention.
U.S. Pat. No. 4,754,555 (Stillman) shows a coplanarity inspection method.
A principle objective of the invention is to improve Ball Grid Array device throughput by providing a method of assuring contact ball coplanarity that does not delay or impact device throughput.
In accordance with the objectives of the invention a new method is provided for assuring contact balls coplanarity. The process of coplanarity inspection is integrated with the current processing step of BGA device singulation, thereby eliminating the need for a separate processing step that is typically required for the coplanarity inspection.